来自《耶鲁大学开放课程：基础物理 第5集 第5集》

• 
  
  If you had two,
  如果有两个驱动力
• 
  
  each will generate its own potential when you do the integral.
  你求积分时就会得到它们各自产生的势能
• 
  
  Then, we may then say E2 - E1;
  然后 我们可以说 E2 - E1
• 
  
  that's the same as (K2 + U2) - (K1 + U1)
  也就是 (K2 + U2) - (K1 + U1)
• 
  
  is equal to the work done by friction.
  等于摩擦力做的功
• 
  
  Work done by friction is the shorthand for that integral
  摩擦力做的功是这个积分式的简写
• 
  
  which you can do only--
  要这个积分式成立
• 
  
  you must divide the motion involved into pieces.
  你得给物体运动分段
• 
  
  For example,
  举例来说
• 
  
  if you have a mass that swung to the left
  如果有个物体先摆到左边
• 
  
  then turns around and goes back to the right,
  然后再向右运动
• 
  
  of course, again, it won't come back to the original A,
  当然它不会回到起始位置 A